The present invention relates to a method and a device for controlling a rock drilling machine where the rock drilling machine comprises an impact device for exerting a drilling tool for impacts, a rotation motor for rotating the drilling tool and a feed motor for feeding the drilling tool toward a ground. The ground can be rock, earth or other material for which rock drilling machines normally are used.
In a previously known method the pressure to the rotation motor is kept constant up to a first predetermined value. This value corresponds, for a given rotation motor, to a predetermined tightening moment for the thread joints in the drill string. When this predetermined value is passed the pressure to the feed motor is reduced in order to keep the pressure to the rotation motor substantially constant. One desires to keep the braking moment substantially constant in order to keep the thread joints sufficiently tightened for efficient drilling at the same time as the tightening is not allowed to be so hard that great difficulties , are obtained at subsequent loosening of the thread joints. If the pressure to the rotation motor in spite of this increases to a second predetermined value the pressure to the impact device is changed from full drilling pressure to collaring pressure. This occurs instantaneously. This causes a risk for jamming since the sudden decrease of the impulse to the impact device causes an instantaneous increase of the feed force.
The present invention, which is defined in the subsequent claims, aims at avoiding the above mentioned jamming problems. This is achieved by controlling the pressure to the inlet of the impact device inversely proportionally to the pressure to the inlet of the rotation motor when the pressure to the inlet of the rotation motor exceeds a second predetermined value.